As packet-switched communication systems slowly replace traditional circuit-switched communication systems, multimedia services are being developed to operate over packet-switching infrastructure. Some of these multimedia services are effectively broadcast/multicast services, in that the bulk of the media, or bearer traffic, flows in one direction. Examples of these services include audio services, such as Internet radio, and audio/video services, such as Internet Protocol Television (IPTV).
Broadcast/multicast services place additional requirements on packet-switching networks. Not only do these services usually consume a considerable amount of capacity, they also may require a special communication channel structure so that the media can be delivered efficiently from a limited number of sources to a potentially enormous number of client nodes.
For example, in wide-area cellular networks, a single broadcast/multicast channel can be employed to deliver media from one source to tens, hundreds, or thousands of client nodes. Such an arrangement is far more efficient than requiring each client node to listen on a dedicated unicast channel.
However, communication networks in general, and wireless networks in particular, are subject to impairments that can lead to packet corruption and packet loss. For instance, a wireless signal can suffer from various types of attenuation, reflections, and/or interference. In order to address these problems, and to reduce the effective packet loss rate of a channel, forward error correction (FEC) schemes may be used.
In general, FEC schemes permit a transmitting device, such as an access node, to transmit multiple copies of part or all of a full packet to a client node. Some copies may include FEC coding, for example XOR coding, Reed-Solomon coding, or turbo codes. If a client node receives a packet with errors, it may be able to apply FEC codes contained within the packet, or contained within packets it has previously received or will subsequently receive, to properly decode the packet. In this way, packet error rates are reduced.
One particularly effective method of implementing FEC is through the use of hybrid automatic repeat request (ARQ). Using hybrid ARQ, an access node that seeks to transmit a full packet instead transmits a series of one or more subpackets. Each of these subpackets may be derived from the full packet, and therefore may contain copies of part or all of the full packet. Each subpacket may also contain an extent of FEC coding. The client node transmits a negative acknowledgment (NACK) for each subpacket it receives until the client node can decode the full packet from the series of subpackets. Once the client node is able to decode the full packet, it may transmit an acknowledgment (ACK) to the access node.
The use of hybrid ARQ may be able to reduce packet error rates of 10% or more to a negligible level. However, hybrid ARQ requires that one or more feedback channels exist so that each client node can transmit an ACK and/or NACK message to the access node. In a broadcast/multicast system with a large number of receivers, such feedback channels can become quickly overloaded with ACK and/or NACK messages. Additionally, these feedback channels may consume precious and limited wireless network capacity, while the implosion of ACKs and/or NACKs arriving at the access node from a large number of client nodes could overwhelm the access node's message processing ability.